Rollup merge of #73380 - pnkfelix:make-bootstrap-help-print-num-cpus, r=Mark-Simulacrum
Rollup merge of #73380 - pnkfelix:make-bootstrap-help-print-num-cpus, r=Mark-Simulacrum

Add more info to `x.py build --help` on default value for `-j JOBS`.

graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust